New directions for high-Q microcavities

Summary form only given. High-Q performance in microcavities relies upon use of low absorption dielectrics and creation of smooth dielectric interfaces. For chip-compatible devices, silica has the lowest intrinsic material loss [1]. Microtoroid resonators combine this low material loss with a reflow technique in which surface tension is used to smooth lithographic and etch-related blemishes [2]. At the same time, reflow smoothing makes it very challenging to fabricate larger diameter UHQ resonators and likewise to leverage the full range of integration tools and devices available on silicon. The devices reported here attain Q factors as high as 875 million using only conventional semiconductor processing methods [3]. Moreover, the best Q performance occurs for diameters greater than 500 microns, a size range that is difficult to access using microtoroids on account of the limitations of the reflow process. Fabrication control of the free-spectral range to 1:20,000 is also demonstrated, opening the possibility of precision repetition rate control in microcombs [4] or precision spectral placement of modes in certain nonlinear oscillators [3,5,6]. As an application, the use of these devices in microcombs [5] and also for microwave generation by stimulated Brillouin scattering will be described [6]. For the latter, a microwave synthesizer with performance comparable to mid-range commercial systems is demonstrated. Finally, the resonator micro-fabrication process also lends itself to creation of record low loss waveguides on chip. Delay lines as long as 27 meters will be described in which waveguide loss is less than 0.1 dB/m [7].